I suspect they've been planning this and experimenting for many months. Along with the large existing test suite, they have lots of tooling for parallelizing agents and an unlimited token budget. So don't feel too bad..
If I was the bun maintainers I wouldn't want to have to maintain a language fork either, when there are viable alternatives.
Now with LLM's getting stronger and stronger a rewrite like this is getting more and more viable, and it looks like they are just approaching this like the "how do you eat an elephant? One bite at a time"-method.
Not sure why people thing this is an elaborate PR campaign, we already know LLM's are increasingly powerful for coding tasks.